Interlocking track circuits

ABSTRACT

An interlocking track circuit and cab signaling arrangement with particular utility for rapid transit applications. The interlocking has five separate track circuits, one for the two crossover tracks and two for each of the main line tracks. Each of the main track circuits is an audio frequency track circuit which does not require insulated joints and which has small loop transmitters. As a result, no supplemental propulsion current return cables are required. Furthermore, the cab signaling arrangement employs the same relatively small loops for transmission purposes obviating the necessity for the large loops required in the prior art. A shorting bar is provided at each of the main line boundaries of the interlocking to prevent signal spillover. The track circuit transmitter is arranged, in relation to a shorting bar, such that a shorting bar failure is self detecting. The low impedance center bond on the main line track circuit provides close shunting sensitivity. The audio frequency main line track circuits increase broken rail detection capability and provides a balanced system which will confine cab signal currents to the intended area.

United States Patent 1191 West et a1.

1 1 INTERLOCKING TRACK CIRCUITS [75] Inventors: Jon F. West, Palmyra; Klaus H.

Frielinghaus, Rochester, both of N.Y.

[73] Assignee: General Signal Corporation,

Rochester, NY.

22 Filed: Sept. 7, 1973 211 Appl. No.: 395,110

Primary ExaminerM. Henson Wood. Jr.

Assistant ExaminerReinhard J. Eisenzopf Attorney, Agent, or FirmPollock, Philpitt & Vande Sande Aug.5,1975

[57] ABSTRACT An interlocking track circuit and cab signaling arrangement with particular utility for rapid transit applications. The interlocking has five separate track circuits, one for the two crossover tracks and two for each of the main line tracks.

Each of the main track circuits is an audio frequency track circuit which does not require insulated joints and which has small loop transmitters. As a result, no supplemental propulsion current return cables are required. Furthermore. the cab signaling arrangement employs the same relatively small loops for transmission purposes obviating the necessity for the large loops required in the prior art. A shorting bar is provided at each of the main line boundaries of the interlocking to prevent signal spillover. The track circuit transmitter is arranged, in relation to a shorting bar, such that a shorting bar failure is self detecting. The low impedance center bond on the main line track circuit provides close shunting sensitivity.

The audio frequency main line track circuits increase broken rail detection capability and provides a balanced system which will confine cab signal currents to the intended area.

7 Claims, 5 Drawing Figures PATENTEU AUG 5197s SHEET mmfs INTERLOCKING TRACK CIRCUITS BACKGROUND OF THE INVENTION In any modern railroad environment, and especially in rapid transit applications, there are four major functions that are carried out, at least in part, by the track rails and equipment associated therewith. The first major function is that of propulsion current return. In the majority of electrically powered trains, a third rail is provided from which propulsion current is drawn. The propulsion current, after energizing the motors and other train-carried equipment, is returned to the rails to complete the power supply to and from a substation. The second major function is that of train detection, that is, location of the train in some specific portion of the track territory. This function is normally carried out by means of a track circuit, a number of which are conventionally used in different applications. Their common characteristic, however, is that of detecting the presence, or absence, of a train. The third function is that of providing cab signaling information to the train. In some instances this cab signaling information is provided from cab signaling current flowing in the track rails, and in other instances, cab signaling information may be provided via current flowing in loops placed adjacent the track rails. The fourth and last function is that of broken rail detection, i.e., a break in the rail gives the appearance of an occupied track section.

In carrying out these four major functions, their requirements, at times, dictated conflicting arrangements. This is especially true in arrangements associated with interlockings.

For purposes of this description, a typical interlocking territory comprises a pair of mainline tracks each interconnected by two intersecting crossover tracks. In this configured interlocking territory there are five areas of interest of which must be defined by separate track circuits. One area consists of the two crossover tracks so that information is available as to when a train enters one of the crossover tracks. The other four areas are distributed, two on each of the mainline tracks. Depending upon the prevailing direction of traffic on each mainline, one of these is an entering section and the other is a leaving section for the interlocking. It is extremely important that precise definition exists between the entering and leaving sections on the mainline so that a train in the entering section, is not detected as being the leaving section. The reasons for this will become apparent as the description proceeds.

In the prior art, this precise definition of the track circuits on the main line was provided using single rail insulated joint track circuits. This circuit provided adequate shunting sensitivity and also enabled good detection of insulated joint breakdown. This was important since it was the insulated joint which differentiated the entering and leaving sections and its condition was therefore important to proper operation of the track circuits. However, use of these single-rail track circuits can impede the performance of the propulsion current return function. The propulsion current return requirement is especially severe in rapid transit applications with its high speed and close headway characteristics. The high speed ability of the trains requires large amounts of propulsion current. The close headway nor mally encountered in rapid transit applications may re sult in a number of trains on a main line between substations. The propulsion current from each ofthe trains must then be handled by the track rails. Insulating one of the main line track rails disables it for purposes of propulsion current return. Absent some other arrangement then, only half the propulsion current return capacity is available through an interlocking. This situation can be intolerable and therefore supplementary propulsion current return cables were thought necessary to parallel the insulated mainline track rail so as to provide the necessary propulsion current return capacity. The disadvantage of this arrangement is, however, the high cost of the propulsion current return cable. In a single rapid transit system, the cost of this cable alone, apart from installation and maintenance expenses, can easily exceed half a million dollars.

As a further corollary of the use of insulated singlerail track circuits in the interlocking on the main line, cab signaling current could not be transmitted in the track rails. Therefore, supplementary cab signaling loops, one for each track section, were provided. Each of these loops was coextensive with the track section, and as a result, was quite large. This meant that the loops were difficult to install, had to be custom engi neered for each location, and custom installed for each location. Furthermore, the large size of these loops made them difficult to drive. Finally, the large loops were a continuing maintenance problem. Furthermore the-single rail track circuits provide broken rail detection in the signal rail but not in the propulsion rail.

SUMMARY OF THE INVENTION The present invention obviates the difficulties encountered with the arrangements referred to above. Single rail insulated track circuits on the main line are replaced by audio frequency track circuits. The elimination of the single rail insulated track circuit eliminates the insulated joint in the center of the interlocking, on the main line, Consequently, both rails of the main line are available for propulsion current return. As a result, the supplemental propulsion return cable can be eliminated. The audio frequency track circuit comprises a transmitter consisting of a small inductive loop coupled to the rails of the main line and also coupled to a shorting bar which is placed across the main line at each of the four limits of the interlocking territory. The intermediate end of each of the audio frequency track circuits on the main line is defined by a track bond which, in the preferred embodiment, provides a low impedance to the track circuit currents to minimize coupling between adjacent track circuits. The bond comprises a portion of the receiver for the track circuit. Cab signaling circuits can be introduced at two points in the track circuit, depending upon traffic direction. At one end, the cab signaling current is introduced by the same inductive loop used for transmitting track circuit current. At the other end, a supplementary cab signal inductive loop is provided for transmitting cab signaling information in the opposite direction. These supplementary cab signal loops are located sev eral feet from the center bond so that the cab signal is coupled into the rail.

The cab signaling and track circuit arrangements for the crossover tracks are similar to the prior art in that cab signaling information is transmitted, in the crossover, via a large inductive loop and the track circuit comprises single rail insulated joint track circuits. Although this reduces the propulsion return capability of the crossover tracks, this lower propulsion current return capacity can be tolerated on the crossover track for a number of reasons. In the first place, the propulsion return currents flowing in the crossover track are derived only from the train which is on the crossover track. Secondly, the speed of trains on the crossover is limited to approximately 22 miles per hour minimizing the propulsion current required. Also since the amount of traffic over the crossover track is a small fraction of the mainline track, restrictions on the crossover traffic will not significantly effect the over-all traffic on the rail system.

BRIEF DESCRIPTION OF THE DRAWINGS In describing the invention reference will be made to the several drawings related to this application, in which like reference characters identify identical apparatus and in which:

FIG. 1A is a schematic drawing of a conventional single-rail insulated track circuit;

FIG. 1B is an illustration of the drawing convention used to represent the track circuit of FIG. 1A;

FIG. 1C is a schematic showing of a prior art cab signaling and track circuit arrangement for an interlocking;

FIG. 2 is a schematic showing of the cab signaling and track circuit interlocking arrangement of the present invention.

FIG. 3 is a representation more nearly to scale of the interlocking shown in FIG. 2.

DETAILED DESCRIPTION OF THE DRAWINGS Before describing the arrangement of the present invention, reference will be made to the prior art to aid in pointing out the differences between this invention and the prior art. However, before discussing the prior art, the conventional AC single-rail track circuit will be discussed along with a showing of a schematic representation thereof.

FIG. IA illustrates a conventional single-rail AC track circuit. In FIG. 1 the track rails 5 and 6 represent a portion of railroad trackway. The extent of the track circuit is defined by the insulated joints 7 and 8 in the rail 5.

A source of AC power is connected to the primary winding of a transformer 9, whose secondary is connected across the track rails 5 and 6 in the vicinity of the insulated joint 7 through an adjustable resistor 12. At the opposite end of the track circuit, adjacent the insulated joint 8, an AC relay 10 has one winding 10-1 connected across the rails through an adjustable resistor 11. Winding 10-1 is referred to as the track winding. A second winding of AC relay 10, local winding 10-2 is connected to the source of AC power. The relay vane 10-3, is operated to one of two positions depending upon the voltage at winding 10-1 and the phase relationship between the voltage at winding 10-1 and the voltage at winding 10-2. In the absence of a train, there is almost a 90 phase difference between these voltages and, through the absence of any axles shorting out winding 10-1, the voltage impressed thereon is relatively high. When a train enters the section of track defined by insulated joints 7 and 8, the difference in phase between the voltages at windings 10-1 and 10-2 decreases sharply as well as the voltage at the track winding 10-1. As a result, the relay vane 10-3 drops out.

This indicates the presence of a train in the section of track protected by the track circuit.

FIG. 1B is a schematic representation of the track circuit shown in FIG. 1A. The track rails 5 and 6 have insulated joints 7 and 8 therein. The darkened rail section between the insulated joints 7 and 8 represents the AC track circuit, transformer and AC relay.

Referring to FIG. 1C, there is represented therein a typical prior art track circuit, cab signaling, and propulsion current return arrangement for an interlocking. The interlocking couples mainlines 15 and 16. The interlocking territory per se, in mainline 15 is defined between insulated joints 17 and 17' on one hand and 18 and 18' on the other hand. With respect to mainline 16, the interlocking territory lies between insulated joints l9 and 19' on one side and 20 and 20' on the other side. The interlocking further consists of crossover tracks 21 and 22 connecting mainlines 15 and 16.

As shown in FIG. 1C, crossover track 21 has singlerail track circuits 21' and 21". Crossover tracks 22 has single-rail track circuits 22' and 22". In actual practice, the single-rail track circuits 21', 21", 22', and 22" are connected as one single track circuit, with one source of power and one AC relay. As a result, when a train is present on either crossover track 21 or crossover track 22, the single AC relay is dropped out indicating the presence of a train on one of the crossover tracks. Specific indication of which crossover track is occupied is not necessary as this information can be deduced from a knowledge of the prevailing traffic direction which is, as is well known, represented by the state of a relay or other bistable element.

Four additional single-rail track circuits are shown, two on each mainline. Rail 15-1 has AC track circuits 23 and 24. AC track circuit 23 extends between insulated joint 17 and insulated joint 25, and AC track circuit 24 extends between insulated joint 25 and insulated joint 18.

Rail 16-2 also comprises two AC track circuits, 26 and 27, the boundaries of which are respectively insulated joint 19' and 28 and insulated joint 28 and 20'. Therefore, for purposes of train detection, there are five different track sections in the interlocking, two on each mainline and the interconnecting tracks.

In order to provide cab signaling information for the trains, a plurality of loops are also included which are conventionally conductors laid adjacent the rails. Coextensive with track circuit 23 is a loop 3A, coextensive with track circuit 24 is a loop 1A, coestensive with track circuit 26 is a loop 18, and coextensive with track circuit 27 is a loop 38. A loop 3R is associated with crossover track 21 and a loop 1R is associated with crossover track 22. Cab signaling information is transmitted in the form of current in the desired loop which is inductively coupled to cab signal receivers aboard the train.

Adjacent the ends of the interlocking territory track bonds 29 through 32 are illustrated. As shown illustratively in FIG. 1C, the bonds 31 and 32 are coupled by cross-bonding connector 37 to equalize the propulsion return currents.

The propulsion return currents flowing in rails 15-1 and 16-2 cannot pass through the interlocking inasmuch as these rails have insulated joints 25 and 28. Therefore, to compensate for this reduced propulsion current return capacity, supplementary propulsion current cables 33, 34, 35, and 36 are provided. Since FIG.

1C is not drawn to scale, the length of the supplementary propulsion current return cables is not apparent. ln actual practice, these cables would span approximately 80% of the length of the interlocking and thus they are substantial in length.

Some of the problems with the arrangement illustrated in FIG. 1C should be apparent from mere inspection. In the first place, each of the mainline sections has two extensive loops in them. Each of these loops must be specially engineered and installed for the particular track section and they must be field tuned. Once installed the loops are difficult to drive since they are high impedance devices and require high powered driving equipment and furthermore the loops are a continuing maintenance problem. Furthermore, the supplementary propulsion return cables are generally copper and therefore relatively expensive in addition to being a further field installation and maintenance problem. In addition, the single-rail AC track circuits utilized exhibit virtually no broken rail protection in the propulsion return rail, that is, in rails -2 and 16-1. In addition the track circuit, being a single rail current, is unbalanced and as a result cab signal loop current will induce currents in the rails which could enter portions of the track for which the cab signaling current is not intended. FIG. 2 is a schematic showing of the interlocking with track circuits and cab signaling arrangements in accordance with the principles of the present invention, As in FlGv 1C, mainlines 15 and 16 and crossover tracks 21 and 22 are shown. As before, the interlocking is defined between insulated joints 17, 17', 18, and 18', 19 and 19, and 20 and 20'. Outside of the interlocking, conventional audio-frequency track circuits are shown. Conventional audio-frequency track circuit transmit and receive bonds are shown as 54 through 61. As in conventional in utilizing audio-frequency track circuits adjacent track circuits are energized with currents of different frequency. That is, the frequency in the track circuit for which receiver 60 is operative is different from the frequency in the track circuit for which receiver 54 is operative.

The crossover tracks 21 and 22 have the same singlerail AC track circuits as in the prior art as well as the cab signaling loops 3R and IR respectively. The mainline arrangements within the interlocking are, however, different from the prior art.

Adjacent the insulated joints which define the interlocking, and inside the interlocking, shorting bars 50 through 53 are provided. These are coupled to the conventional bonds 54 through 57 to provide a propulsion current path bypassing the insulated joints. Each of the mainline tracks 15 and 16 is, within the interlocking, divided into two adjacent track circuits, which are mirror images of one another. For purposes of this description we will discuss one such track circuit.

An inductive loop 44 acts as a transmitter for track circuit current for the track circuit between insulated joints 19 and 19' and receiver 49. The same transmitter 44 also transmits cab signaling information for train moves from right to left. For train moves in the opposite direction, inductive loop 45 acts as a transmitter of cab signal information.

lnductive loop 44 is tightly coupled to the adjacent shorting bar 51 and is also coupled along its length with the adjacent rails 16-1 and 16-2. For reasons which will become apparent, however, the length of this loop in the direction of the track rail is such that 50 percent or more of the coupled energy from the loop is coupled into the shorting bar 51. In a preferred embodiment, the length of the loop 44 along the length of the track rails is on the order of 2 feet. The inductive loop 45, however, is spaced from the track bond and receiver 49 by a distance sufficient to ensure that the cab signaling current which the loop 45 induces in the rails 16-1 and 16-2 proceeds to the left in FIG. 2 and a minimum amount of cab signaling current is induced in the receiver and bond 49. To this end, inductive loop 45 is spaced from the receiver and bond 49. In a preferred embodiment, the minimum spacing between the portion of inductive loop closest to the receiver and bond 49 is at least 2 feet.

The loops 46 and 47 in the adjacent track circuit on mainline 16, the shorting bar 53, loops 40, 41, 42, 43 and shorting bars 50 and 52 are arranged in a like manner.

As is conventional in adjacent audio-frequency track circuits, the track circuit current in the circuit between joints 19 and 19' and bond 49 is different from the track circuit current in the circuit between the joints 20 and 20' and bond 49.

The audio-frequency track circuits for mainline track 15 are arranged in a similar manner. The center bonds in each of mainlines 1S and 16, 48 and 49 are required to minimize the coupling from one track circuit into the next and also must be capable of withstanding high inbalanced propulsion return currents due to single-rail track circuits in the crossover. The reason for this will become apparent in discussing FIG, 3 when the propulsion current return connections between the crossover tracks and the mainlines are illustrated and discussed in detail. The requirement that the bonds 48 and 49 minimize coupling from one circuit to the next is necessary to obtain precise definition of shunting. That is, with a train in the section between receiver 49 and joints 20 and 20, the track circuit between receiver 49 and joints 19 and 19', should not falsely indicate train occupancy due to the same train.

The bonds, 48 and 49, are untuned in contrast to conventional audio-frequency track circuit bonds. They present an extremely low impedance, about 0.06 ohms to the track circuit current. For track circuit indications, two tuned receivers are each connected to the bonds. One receiver responds to track circuit signals produced by transmitter 40 and monitors the track between joints 17, 17 and bond 48, and a second receiver responds to track circuit signals produced by transmitter 43 and monitors the track between bond 48 and joints l8 and 18'. A similar arrangement is used on the other mainline 16.

From the foregoing description the manner in which the propulsion current return is handled in the interlocking should be apparent. In the first place, no supplementary propulsion return cables are required. The reason for this is that both rails in both mainlines 15 and 16 are utilized for propulsion current return. The propulsion current flowing in mainline tracks 15-1 and 15-2, outside the interlocking, flows through the track bond 54 and the shorting bar 50 into the rails 15-1 and 15-2 inside the interlocking. This propulsion current then flows down the rails in the mainline 15 inside the interlocking territory and out through shorting bar 42 and track bond 57. Since both track rails are available for propulsion current return inside the interlocking,

no supplementary propulsion current return cables are required.

The reason that both track rails can be utilized for propulsion current return is that there are no insulated joints in the mainline inside the interlocking territory. The reason for this is the use of audio-frequency double-rail track circuits. The inerlocking signaling and control arrangement shown in FIG. 2 also does away with the requirement for long loops for cab signaling currents. The arrangement in FIG. 1C could not utilize the rails for cab signaling current flow since this requires crossbonding of the rails and that could not be accomplished using the single-rail AC track circuits. The shorting bars 50 through 53 assure minimum spillover of cab signaling current inside the interlocking territory to the rails outside the interlocking territory regardless of the condition of the insulated joints which define the interlocking territory. The extremely low impedance of these shorting bars, on the order of 0.001 ohms accomplishes this function. Furthermore, the arrangement as described above, provides for selfdetecting open shorting bar failures. Since 50 percent of the track circuit current is coupled through the shorting bar, if the shorting bar does become open circuited, this results in a decrease in track circuit current of at least 50 percent. Furthermore, open circuiting of the shorting bar will increase the impedance of the track circuit thus further reducing the track circuit current flowing in the rails. In a preferred embodiment with an open shorting bar. the track circuit voltage available at the receiver was reduced to one-eighth of the value normally available. Thus, if a shorting bar does become open circuited, this failure is selfdetecting.

ln utilizing the apparatus shown in FIG. 2, further ap paratus is required, to respond to train presence indications for initiating the flow of cab signaling current in the proper sections of track. Cab signaling current flow is initiated in response to train movements and, as is conventional in the art, the indications given depend upon traffic conditions in the direction of movement of the train. In a straight through move, that is, where the train does not cross over from one mainline track to the other, cab signaling current is confined to the mainline on which the train traveling. For example, for a straight through move on mainline I5 from left to right, cab signaling flow is transmitted through transmitters 41 and 43. When the receiver located at the bond 48, responsive to track circuit current from transmitter 43, drops out, indicating the train is in the section between the bond 48 and the joints l8 and 18', the flow of cab signaling current from transmitter 41 ceases. When the receiver associated with the bond 57, just outside the interlocking, drops out indicating the front of the train has passed through the interlocking, then the cab sig naling current transmitted by transmitter 43 ceases. Straight-through moves in the other direction on mainline 15, and in both directions on mainline l6 occur in substantially the same manner.

For a crossover move, of course, one of the loops 3R and IR must be energized to transmit cab signaling current to the train when it reaches the crossover track, either 21 or 22. We will explain the flow of cab signaling current in response to train presence indications for a movement from left to right on mainline I5 which uses crossover track 21 to enter mainline l6. Cab signaling current flow is initiated via transmitters 41 and 3R as the train enters the interlocking territory. When the train presence is indicated on the crossover track 21, such as when the 60 cycle AC track circuit associated with crossover tracks 21 and 22 drops out, the transmission of cab signaling current from the transmit ter 41 ceases. At the same time, transmitter 47 is energized to transmit cab signal current. When the train proceeds across the crossover track 21 and exits the interlocking by passing bonds 20 and 20' on the mainline I6, the receiver associated with the bond 56 will drop out. This indicates that the front of the train has passed out of the interlocking. At that time, the cab signal current transmitters 3R and 47 cease transmitting cab signaling information.

Crossover moves using crossover track 21 in the opposite direction, and crossover moves using crossover track 22 in both directions, are accomplished in much the same manner as has been explained above.

FIG. 3 substantially duplicates FIG. 2 except for the scale of the representation. Whereas FIG. 2 is drawn schematically, FIG. 3 is drawn more nearly to scale in order to show the length of the various sections in the interlocking. In addition to illustrating the apparatus shown in FIG. 2, FIG. 3 also illustrates the propulsion current cross-bonding between the main line and the crossover tracks. In particular, cross-bonds 71 through 86 are shown to complete the propulsion current return path from the various insulated sections of the crossovers 21 and 22 to the main lines 15 and 16.

From the showing of FIG. 3, the difference in size between the track circuit loop transmitters used in FIG. 3 as compared to the size of the loops that would have been necessary had the prior art been followed should be apparent. Reference to FIG. 1C will show that the loop 3A, for instance, would be much larger, extending from the insulated joints l7 and 17' to the cross-bond 48 as compared to the loops 40 and 41 actually employed in the arrangement according to the present invention.

In order to illustrate the length of the propulsion current return cable that has been eliminated, reference numeral 90 is employed to designate the position of the interlocking end point. Reference numeral 91 identifies the point at which the propulsion current return cable would be connected to one of the mainline tracks had such cable been necessary. Thus, the length from reference numeral 90 to reference numeral 91 identifies the length of propulsion current return cable for a track section which has been eliminated. Of course, the entire interlocking would require four cables each as long as the distance between reference numerals 90 and 91.

It has been pointed out above that the audiofrequency track circuits used in the mainlines l5 and 16, within the interlocking, provide greater broken rail protection than did the single-rail AC track circuits utilized in the prior art. Reference to FIG. 1A will be of aid in understanding the reasons behind this. As was explained with reference to FIG. IA, the condition of the relay vane 10-3 depends upon the voltage and the winding 10-1 and the phase difference between the signal in the winding 10-1 and the signal in the winding 10-2. It is entirely possible that rail 6 can become broken and thus substantially open circuited, in the area between joints 7 and 8. There will be, however, sufficient current flow around this open circuit through crossbonding to the adjacent track to maintain a sufficient voltage at winding 10-1 to maintain vane I0-3 in the energized position. Thus, in this case, the broken rail 6 has not been detected. To the contrary, however, the audio-frequency track circuit is a closed loop circuit defined by the bonds (or shorting bar) at each end and is entirely dependent upon the voltage received by the track circuit receiver, As a result, a broken rail increases the impedance in the track circuit to a sufficient extent so that the receiver indicates the presence of a train when actually no train is present. Thus, the broken rail is self-detecting.

FIG. 3 is also helpful in pointing out the necessity for sharp shunting definition in the interlocking territory. For purposes of this discussion we can assume that a train on mainline 16 moving from left to right makes a crossover move to mainline over crossover track 22. Because of the crossbonding, particularly at 73 and 74, the train on the crossover track 22 will appear as if it were on the mainline 16 at least up to the points of the bond connections 73 and 74 to the mainline 16. Thus, to the receiver connected to the cross-bond 49, it will appear as if a train were standing on mainline 16 at the point. Thus the shunting definition of the track circuit between insulation joints and 20 and the bond 49 should be adequately sharp to ensure that a train standing in this position, at the connection points of the bonds 73 and 74, does not drop the track circuit defined by bond 49 and joints 20 and 20'. If this occurs, it will appear to the control logic as if a train were in the track section between insulator joints 19, 19' and the cross-bond 49 and a second train were in the track circuit between the cross bond 49 and the insulator joints 20 and 20'. As a safety measure, if this condition occurs, all cab signaling will cease, in the interlocking. Thus, a single train traveling through the interlocking would be self stopping in that it would be detected in two different track sections at the same time. Therefore, the shunting definition of the mainline interlocking track circuit must be sufficiently sharp to prevent this.

In the preferred embodiment, this shunting sensitivity is obtained by providing low impedance cross bonds such as 48 and 49, and in this context, low impedance is on the order of 0.06 ohms. However, these cross bonds, 48 and 49 could, alternatively, be made high impedance devices and still obtain the necessary shunting sensitivity. In order to provide the necessary shunting sensitivity with a high impedance bond, current detectors must be provided, to supplement the bond. When the voltage at the bond decreases, it is necessary to know whether there is any substantial flow of current past the bond. That is, if the current is flowing from the transmitter on one side of the bond to a low impedance on the other side of the bond. When a low signal voltage at the bond occurs by reason of a low impedance on the side of the bond opposite from the transmitter, then this condition should not be responded to. The reason for this is that the low impedance causing the lowered signal voltage at the receiver is presumably as a result of wheels and axles of a train not between the bond and its transmitter, but on the other side of the bond. To obtain adequate shunting sensitivity this condition should not be responded to inasmuch as the train whose wheels and axles are causing the low impedance is not within the circuit that the receiver is to respond to. Only when the low signal voltage at the receiver oc curs with no substantial flow of current past the bond, should the receiver respond to indicate the presence of a train between the transmitter and the bond. In this manner, adequate shunting sensitivity can be obtained with a relatively high impedance bond which is conventional in audio-frequency track circuits.

Thus, applicant has provided an interlocking which avoids the major disadvantages inherent in the prior art. The necessity for propulsion return cables has been eliminated, the large loops in the mainline required for cab signaling current have been eliminated while at the same time obtaining adequate shunting definition, broken rail detection, and minimizing signal spillover across the insulated joints defining the interlocking.

What we claim is:

1. In an interlocking territory including a pair of main-line tracks interconnected by at least one crossover track intersecting both said main-line tracks, said interlocking territory defined between insulated rail joints in each of said main-line tracks, an improved signaling and control arrangement including uninsulated track circuits in said main-line tracks within said interlocking territory, comprising,

a single rail track circuit on said crossover track,

a pair of track circuits on each of said main-line tracks within said interlocking, each of said track circuits comprising a transmitter inductively coupled to said track and a track bond common to two of said track circuits,

and a plurality of track bond means connected across said main-line tracks outside said interlocking territory and means connecting each of said track bond means to said main-line tracks inside said interlocking territory.

2. The apparatus of claim 1 in which each of said track circuits has at least one cab signal transmitter inductively coupled to said tracks.

3. The apparatus of claim 2 wherein said cab signal transmitter is located at one boundary of each track circuit and said track circuit transmitter is located at another boundary,

4. The apparatus of claim 3 in which said cab signal transmitter is located at the track bond boundary and said transmitter is located at said shorting-bar boundary of said track circuit.

5. The apparatus of claim 4 in which said transmitter is adjacent and tightly coupled to said shorting bar and said cab signal transmitter is spaced from said track bond.

6. The apparatus of claim 1 wherein said track bond presents a low impedance to track circuit current.

7. The apparatus of claim 1 wherein said means connecting each of said track bond means to said main-line tracks inside said interlocking territory includes a shorting bar, each of said shorting bars conductively coupled to said main-line tracks inside said interlocking territory,

each of said transmitters inductively coupled to said main-line tracks, at least partially, through one of said shorting bars placed adjacent said transmitters. 

1. In an interlocking territory including a pair of main-line tracks interconnected by at least one crossover track intersecting both said main-line tracks, said interlocking territory defined between insulated rail joints in each of said main-line tracks, an improved signaling and control arrangement including uninsulated track circuits in said main-line tracks within said interlocking territory, comprising, a single rail track circuit on said crossover track, a pair of track circuits on each of said main-line tracks within said interlocking, each of said track circuits comprising a transmitter inductively coupled to said track and a track bond common to two of said track circuits, and a plurality of track bond means connected across said mainline tracks outside said interlocking territory and means connecting each of said track bond means to said main-line tracks inside said interlocking territory.
 2. The apparatus of claim 1 in which each of said track circuits has at least one cab signal transmitter inductively coupled to said tracks.
 3. The apparatus of claim 2 wherein said cab signal transmitter is located at one boundary of each track circuit and said track circuit transmitter is located at another boundary.
 4. The apparatus of claim 3 in which said cab signal transmitter is located at the track bond boundary and said transmitter is located at said shorting-bar boundary of said track circuit.
 5. The apparatus of claim 4 in which said transmitter is adjacent and tightly coupled to said shorting bar and said cab signal transmitter is spaced from said track bond.
 6. The apparatus of claim 1 wherein said track bond presents a low impedance to track circuit current.
 7. The apparatus of claim 1 wheRein said means connecting each of said track bond means to said main-line tracks inside said interlocking territory includes a shorting bar, each of said shorting bars conductively coupled to said main-line tracks inside said interlocking territory, each of said transmitters inductively coupled to said main-line tracks, at least partially, through one of said shorting bars placed adjacent said transmitters. 